Anyone who has struggled to load a horse onto a trailer understands the inherent challenge and risks.

As prey animals, horses naturally stay away from confined spaces where, in the wild, they might be trapped by predators. While horses’ large eyes allow for nocturnal vision, scientists suspect their capacity for visual detail is reduced in low light. So how do you get horses to load in a relaxed way into the confined, often dimly lit, space of trailers?

Many factors play a role in ensuring a horse walks carefully and pleasantly onto a trailer. A quiet atmosphere with treats and petting for positive reinforcement is a good start. Inexperienced haulers usually prefer to have a friend already loaded to give them confidence as they enter the trailer. Familiar smells of hay and straw inside the trailer also help.

A recent study finds another factor may also affect the chances of a stress-free, injury-free loading experience for both horse and handler—lighting outside and inside the trailer.

“Effects of lighting conditions on the welfare of horses being loaded for transportation,” published in the Journal of Veterinary Behavior in January 2008, suggests that horses are the least comfortable when loading from a well-lit environment into a dark trailer.

The University of Queensland study trained eight yearlings, four colts and two fillies, to walk up a ramp into a partially enclosed trailer using a kind human voice for encouragement, and neck scratching and hay as rewards. The horses, outfitted with heart monitors, were loaded from both bright and dark arenas into both bright and dark trailers.

The horses’ heart rates increased on average by 50 beats per minute in every loading situation, regardless of the lighting outside or inside the trailer. This shows that trailer loading in general caused stress and fear in the horses. The study points out it is likely the horses would have experienced more fear and a higher increase in heart rates had the trailer used been completely enclosed.

Interestingly, the number of refusals and the loading speed remained constant with all the lighting conditions. This suggests the horses’ stress and fear did not escalate when the lighting inside and outside the trailer changed.

However, when loading from a well-lit environment into either a bright or dark trailer, the horses tended to look away from the trailer and lower their heads more. Additionally, when loading from a well-lit environment to a dark trailer, the horses sniffed the ground more, demonstrating the desire to further investigate their surroundings. While not outright displays of fear, these behaviors are indications of apprehension and discontent.

The findings of this study show the importance of opening the trailer windows and turning on the interior lights prior to loading, particularly when the outside environment is bright. Keeping the inside of the trailer flooded with light will help horses stay calm, cooperative, and safe throughout the loading process.

Read the complete study in Journal of Veterinary Behaviour.